Read the "Elements of Style" by William Strunk. It is still one of the classic works regarding English grammar and usage. You can purchase the book in a local bookstore or read it free online (see Resources).
Visit Ask Grammar. This site is sponsored by the Capital Community College in Hartford and features an extensive resource library for the study of advanced grammar. The topics are well organized and presented with easy-to-navigate drop-down menus. This site is worth bookmarking and returning to on a regular basis, for beginners and pros alike. If you can't find the rule you're looking for, you can Ask Grammar. She'll be happy to respond.
Consider ordering the Bluebook of Grammar (see Resources). This is a well-written, comprehensive reference guide on English grammar. The website features free grammar instructional videos, articles, and even quizzes to help you learn advanced grammar. The subjects are systematically laid out and easy to follow.
The English Page (see Resources) features mini-tutorials, an extensive library of grammatical reference materials and dictionaries and a well categorized online lesson plan to help you master English grammar. You don't have to sign up to use the service. Simply visit the website and follow the weekly lesson plans.